1. Field of the Invention
The present invention relates to a measuring apparatus for performing the measurement using an element having its impedance varied depending on the surrounding state.
2. Related Background Art
Conventionally, it has been quite difficult to measure the humidity in a wide range from high to low temperatures at a high precision. This is because the output of a humidity sensor has a high-order .gamma. characteristic as shown in FIG. 2, which makes difficult the linear approximation in a wide range.
It is the common practice to make use of the voltage-current characteristics of a diode for the linear conversion, but to compensate for the difference between the actual sensor characteristic and the diode characteristic, it was necessary to individually adjust a respective combination. The precision, however, is insufficient to make the linear conversion for a wide range of humidity.
In addition, a method has been proposed in which a high-order approximate curve is obtained by the least square method, using a CPU, and a sensor output is substituted thereinto to make the linear approximation, and has been already put to practical use, but in order to make the measurement at a high precision in a wide range, the higher resolution in a measurement circuit is required, thereby requiring higher bits of CPU or A/D converter, or an increase in the processing time of CPU.
However, as shown in FIG. 2, the output of a sensor may vary from 1 K.OMEGA. to 10 M.OMEGA. or greater with respect to the relative humidity of 20 to 95%. If a measurement is tried at a low humidity of 10% or less, a high resistance of 100 M.OMEGA. or greater is anticipated. To have a measurement precision for the relative humidity of 5% or less, a precision (resolution) of 0.3 K.OMEGA. is necessary at 60.degree. C. and near 95%, so that a resolution of not more than 1/330000 is required in a detection circuit, A/D converter and CPU, whereby it was impossible to actually measure the humidity in such a wide range at a high precision.
Also, an apparatus has been proposed in which a humidity sensor having its resistance varied with the humidity and a condenser are connected in series to apply a DC voltage between the humidity sensor and the condenser, whereby the time until the condenser voltage reaches a predetermined value and the sensor characteristic are calculated by using a linear approximate expression.
However, in particular, if the range in the low humidity side is expanded, a problem arises that the resistance of humidity sensor and thus the charging time will increase. For example, at 25.degree. C. and near 10%, the resistance of humidity sensor is 10 G.OMEGA. or greater, whereby even if the capacity of condenser to be charged is decreased to about 30 PF, a charging time as long as about 300 ms is necessary. If the charging time is expanded to such a level, the humidity sensor is effectively equivalent to a state in which a DC voltage is applied, so that the humidity sensor may degrade rapidly due to the deflection of internal ions.